1. Field of the Invention
The present invention relates to an O2 sensor for use in the exhaust system of an internal combustion engine, an apparatus for and a method of controlling the air-fuel ratio in a system where an O2 sensor is incorporated in the exhaust system of an internal combustion engine, and a recording medium which stores an air-fuel ratio control program for such a system.
2. Description of the Related Art
There has been proposed by the applicant of the present application a system for controlling the air-fuel ratio of an exhaust gas, i.e., the air-fuel ratio represented by the concentration of oxygen in an exhaust gas, supplied from an internal combustion engine to a catalytic converter for keeping the output voltage of an O2 sensor disposed downstream of the catalytic converter at a predetermined target value (a constant value) in order to give a required exhaust gas purifying capability to the catalytic converter, which comprises a three-way catalyst or the like disposed in the exhaust system (passage) of the internal combustion engine (see, for example, Japanese laid-open patent publication No. 11-324767 and U.S. Pat. No. 6,188,953).
The proposed system is based on the phenomenon that when the air-fuel ratio of the exhaust gas supplied from the internal combustion engine to the catalytic converter is controlled at an air-fuel ratio state for settling the output voltage of the O2 sensor disposed downstream of the catalytic converter at a predetermined constant value, the rates of purification of CO (carbon monoxide), HC (hydrocarbon), NOx (nitrogen oxide), etc. by the catalytic converter are kept well, i.e., substantially maximum, irrespective of the deteriorated state of the catalytic converter.
A further study made by the inventors of the present invention has revealed the following findings:
An O2 sensor has its output characteristics changed when the temperature of an active element of the O2 sensor, i.e., a sensitive element of the O2 sensor which is held in contact with an exhaust gas, is changed by factors that affect the temperature of the active element of the O2 sensor, such as the layout of the exhaust system of the internal combustion engine, the temperature of the exhaust gas, etc. A change in the output characteristics of the O2 sensor tends to affect the control properties (control response and stability) of the air-fuel control process for keeping the output voltage of the O2 sensor disposed downstream of the catalytic converter at a predetermined target value. This is because such a change in the output characteristics of the O2 sensor varies the sensitivity of the output voltage of the O2 sensor with respect to an air-fuel ratio change in the vicinity of the target value. The change in the output characteristics of the O2 sensor also varies the value of the output voltage of the O2 sensor disposed downstream of the catalytic converter for making well the exhaust gas purifying capability of the catalytic converter, i.e., the value of the output voltage of the O2 sensor to be used as a target value for maintaining the good exhaust gas purifying capability of the catalytic converter.
Therefore, the control properties of the air-fuel control process for keeping the output voltage of the O2 sensor at the target value may possibly be lowered depending on the temperature of the active element of the O2 sensor or the operating conditions or environmental conditions of the internal combustion engine which affect the temperature of the active element of the O2 sensor. Furthermore, if factors that affect the temperature of the O2 sensor, e.g., the temperature of the exhaust gas, are likely to vary while the internal combustion engine is in operation, then it tends to be difficult to keep the desired exhaust gas purifying capability of the catalytic converter even when the output voltage of the O2 sensor is controlled at the constant target value.
It is therefore an object of the present invention to provide an O2 sensor having adequate output characteristics for maintaining the required exhaust gas purifying capability of a catalytic converter.
Another object of the present invention is to provide an apparatus for and a method of controlling an air-fuel ratio to reliably maintain the required exhaust gas purifying capability of a catalytic converter.
Still another object of the present invention is to provide a recording medium which stores a program for enabling a computer to control an air-fuel ratio to reliably maintain the required exhaust gas purifying capability of a catalytic converter.
Prior to describing the present invention in detail, the output characteristics of an O2 sensor will first be described below. An O2 sensor which is disposed in the exhaust passage of an internal combustion engine generates an output voltage depending on the concentration of oxygen in an exhaust gas which is brought into contact with an active element (sensitive element) of the O2 sensor. The active element of the O2 sensor is usually made of a material containing or coated with lead, silver, platinum, etc. (e.g., zirconia (ZrO2+Y2O3)). The O2 sensor has output characteristics (specifically, the characteristics of an output voltage of the O2 sensor with respect to the air-fuel ratio in the exhaust gas which is represented by the concentration of oxygen sensed by the active element) generally referred to as a Z curve.
More specifically, as indicated by the solid-line curve xe2x80x9caxe2x80x9d in FIG. 3 of the accompanying drawings, the output characteristics of the O2 sensor have a segment e1 where the output voltage changes substantially linearly at a relative large gradient with respect to a change in the air-fuel ratio in the exhaust gas (hereinafter referred to as xe2x80x9clarge-gradient segment e1xe2x80x9d), and segments e2, e3 where the gradient of a change in the output voltage with respect to a change in the air-fuel ratio is smaller than the large-gradient segment e1 (hereinafter referred to as xe2x80x9csmall-gradient segments e2, e3xe2x80x9d). The small-gradient segments e2, e3 are present respectively on both sides of the large-gradient segment e1, i.e., respectively in regions that are richer and leaner than an air-fuel ratio range xcex94 corresponding to the large-gradient segment e1. The air-fuel ratio range xcex94 corresponding to the large-gradient segment e1 is a narrow range near a stoichiometric air-fuel ratio. The gradients of the small-gradient segments e2, e3 are much smaller than the gradient of the large-gradient segment e1. The small-gradient segments e2, e3 are joined to the large-gradient segment e1 by respective boundary segments e4, e5 that provide inflection points across which the gradient changes greatly. The output characteristics shown in FIG. 3 are general output characteristics of the O2 sensor. The air-fuel ratio in the exhaust gas according to the above output characteristics, i.e., the air-fuel ratio represented by the concentration of oxygen that is sensed by the active element of the O2 sensor, becomes richer, i.e., the ratio of fuel to air being larger, as the concentration of oxygen is lower, and becomes leaner, i.e., the ratio of fuel to air being smaller, as the concentration of oxygen is higher.
According to the inventors"" knowledge, when the air-fuel ratio in the exhaust gas supplied to the catalytic converter is controlled to keep the output voltage of the O2 sensor with the above output characteristics at a certain constant target value, it is preferable for the purpose of improving the control properties of such an air-fuel ratio control process to have the target value at a level substantially equal to an output voltage at the inflection point e4 at a richer air-fuel ratio, of the two inflection points e4, e5, or specifically to have the target value in an output voltage range in at the inflection point e4.
The above target value control is preferable for the following reasons: At the inflection point e4, the gradient (average gradient) of the output voltage of the O2sensor with respect to the air-fuel ratio is intermediate between the larger gradient of the large-gradient segment e1 and the smaller gradient of the small-gradient segment e2. Thus, the gradient at the inflection point e4 is not too large and not too small, but is an adequate gradient. Stated otherwise, at the inflection point e4, a change in the output voltage (sensitivity) with respect to a change in the air-fuel ratio is not too large and not too small. The small-gradient segment e2 that is contiguous to the inflection point e4 has a certain gradient (xe2x89xa00), allowing the output voltage to be somewhat sensitive to a change in the air-fuel ratio. Generally, when the air-fuel ratio turns leaner, NOx in the exhaust gas tends to increase, and hence it is preferable to make the air-fuel ratio richer quickly. Consequently, when the air-fuel ratio turns leaner, the output voltage of the O2 sensor should preferably change highly sensitively. As a result, it is preferable for the target value to be of a level substantially equal to an output voltage at the inflection point e4, i.e., to have an output voltage range in at the inflection point e4 in the vicinity of the target value.
According to the inventors"" knowledge, furthermore, the output characteristics of the O2 sensor change depending on the temperature of the active element thereof, as shown in FIG. 3. In FIG. 3, the solid-line curve xe2x80x9caxe2x80x9d, a broken-line curve xe2x80x9cbxe2x80x9d, a dot-and-dash-line curve xe2x80x9ccxe2x80x9d, and a two-dot-and-dash-line curve xe2x80x9cdxe2x80x9d represent the output characteristics of the O2 sensor when the active element of the O2 sensor has temperatures of 800xc2x0 C., 750xc2x0 C., 700xc2x0 C., and 650xc2x0 C., respectively. In order to explain the general output characteristics of the O2 sensor as described above, the solid-line curve xe2x80x9caxe2x80x9d typically has its large-gradient segment, small-gradient segment, and inflection points denoted respectively by e1 through e5 in FIG. 3. The other curves xe2x80x9cbxe2x80x9d through xe2x80x9cdxe2x80x9d which also represent the output characteristics of the O2 sensor also have their large-gradient segment, small-gradient segment, and inflection points (which mean exactly the same as with the solid-line curve xe2x80x9caxe2x80x9d) denoted respectively by reference characters that are identical to those of the solid-line curve xe2x80x9caxe2x80x9d. Of each of those other curves xe2x80x9cbxe2x80x9d through xe2x80x9cdxe2x80x9d, the small-gradient segment denoted by e2 refers to a small-gradient segment at richer air-fuel ratios, and the small-gradient segment denoted by e3 refers to a small-gradient segment at leaner air-fuel ratios. The inflection point denoted by e4 refers to an inflection point between the large-gradient segment e1 and the small-gradient segment e2, and the inflection point denoted by e5 refers to an inflection point between the large-gradient segment e1 and the small-gradient segment e3.
As shown in FIG. 3, the temperature of the active element of the O2 sensor affects the output characteristics of the O2 sensor, particularly, the gradient of the large-gradient segment e1 and the level of the output voltage in the small-gradient segment e2 at richer air-fuel ratios. Specifically, the level of the output voltage in the small-gradient segment e2 is basically lowered (and the level of the output voltage in the inflection point e4 is also lowered) as the temperature of the active element rises. More generally, the level of the output voltage in the small-gradient segment e2 changes toward the level of the output voltage in the other small-gradient segment e3 as the temperature of the active element rises. Basically, the gradient of the large-gradient segment e1 becomes more gradual as the temperature of the active element is lower. A comparison between the broken-line curve xe2x80x9cbxe2x80x9d which is plotted when the temperature of the active element is 750xc2x0 C. and the solid-line curve xe2x80x9caxe2x80x9d which is plotted when the temperature of the active element is 800xc2x0 C. indicates that when the temperature of the active element of the O2 sensor is 750xc2x0 C. or higher, the output characteristics of the O2 sensor are substantially constant i.e., changes in the output characteristics of the O2 sensor with respect to changes in the temperature of the active element are small.
The inventors"" knowledge also reveals that when the O2 sensor with the above output characteristics is disposed downstream of a catalytic converter, e.g., a three-way catalyst, and the air-fuel ration in an exhaust gas supplied to the catalytic converter to keep the output voltage of the O2 sensor at a constant value, the rates of purification by the catalytic converter of CO, HC, and NOx in the exhaust gas are correlated to the value of the output voltage of, the O2 sensor, as indicated by a group of solid-line curves or a group of broken-line curves in FIG. 4 of the accompanying drawings. The group of solid-line curves in FIG. 4 shows the relationship between the rates of purification of CO, HC, and NOx and the output voltage of the O2 sensor when the temperature of the active element of the O2 sensor is 650xc2x0 C., and the group of broken-line curves in FIG. 4 shows the relationship between the rates of purification of CO, HC, and NOx and the output voltage of the O2 sensor when the temperature of the active element of the O2 sensor is 800xc2x0 C.
As shown in FIG. 4, the output voltage Vop of the O2 sensor for optimizing the rates of purification by the catalytic converter of CO, HC, and NOx (hereinafter referred to as xe2x80x9cpurification optimizing output voltage Vopxe2x80x9d) differs depending on the temperature of the active element of the O2 sensor. This is because the output characteristics of the O2 sensor change depending on the temperature of the active element of the O2 sensor, as described above. For example, if the temperature of the active element of the O2 sensor is 650xc2x0 C., then the purification optimizing output voltage Vop (650xc2x0 C.) of the O2 sensor is about 0.67[V], and if the temperature of the active element of the O2 sensor is 800xc2x0 C., then the purification optimizing output voltage Vop (800xc2x0 C.) of the O2 sensor is about 0.59[V].
It is to be noted in particular that the purification optimizing output voltage Vop (800xc2x0 C.) of the O2 sensor when the temperature of the active element of the O2 sensor is 800xc2x0 C. is substantially the same as the output voltage in the inflection point e4 of the output characteristics of the O2 sensor (curve xe2x80x9caxe2x80x9d) at 800xc2x0 C.. Since the output characteristics of the O2 sensor are substantially constant when the temperature of the active element of the O2 sensor is 750xc2x0 C. or higher, as described above, the purification optimizing output voltage Vop (750xc2x0 C.) (not shown) of the O2 sensor when the temperature of the active element of the O2 sensor is 750xc2x0 C. is substantially the same as the purification optimizing output voltage Vop (800xc2x0 C.) of the O2 sensor at 800xc2x0 C. Therefore, the purification optimizing output voltage Vop (750xc2x0 C.) at 750xc2x0 C. is substantially the same as the output voltage in the inflection point e4 of the curve xe2x80x9cbxe2x80x9d.
It follows from the above analysis that
(1) for controlling the air-fuel ratio in the exhaust gas to maintain the output voltage of the O2 sensor at a certain target value, it is preferable for the target value and the output voltage in the inflection point e4 of the output characteristics of the O2 sensor to be of substantially the same levels as each other,
(2) the output characteristics of the O2 sensor (particularly the levels of the output voltages in the small-gradient segment e2 and the inflection point e4) can be adjusted or kept constant by controlling the temperature of the active element of the O2 sensor, and
(3) when the temperature of the active element of the O2 sensor which is disposed downstream of the catalytic converter is controlled at 750xc2x0 C. or higher, the purification optimizing output voltage Vop of the O2 sensor and the output voltage in the inflection point e4 of the output characteristics of the O2 sensor are of substantially the same levels as each other.
With respect to (2) above, it is also possible to change the levels of the output voltages in the small-gradient segment e2 and the inflection point e4 by adjusting the contents of materials such as lead, silver, etc. in the active element of the O2 sensor.
The present invention will be described below on the basis of the output characteristics of the O2 sensor as described above. First, an O2 sensor according to the present invention will be described below. In order to achieve the above object, according to the present invention, the O2 sensor is used in a system for controlling an air-fuel ratio in an exhaust gas supplied from an internal combustion engine to a catalytic converter disposed in an exhaust passage of the internal combustion engine for keeping an output voltage of the O2 sensor at a predetermined target value to achieve a predetermined exhaust gas purifying capability of the catalytic converter, the O2 sensor being disposed in the exhaust passage downstream of the catalytic converter for generating an output voltage having a level depending on the concentration of oxygen in the exhaust gas, the output voltage changing with respect to the air-fuel ratio in the exhaust gas which is represented by the concentration of oxygen, at a gradient which switches from a large gradient to a small gradient via an inflection point as the air-fuel ratio turns richer, the O2 sensor having such output characteristics that the output voltage thereof at the inflection point is substantially the same as the target value. The xe2x80x9cinflection pointxe2x80x9d referred to above corresponds to the inflection point e4 shown in FIG. 43. For illustrative purpose, the xe2x80x9cinflection pointxe2x80x9d which will be referred to in the description of the present invention is also denoted by xe2x80x9ce4xe2x80x9d.
With the above arrangement of the present invention, since the target value for the output voltage of the O2 sensor and the output voltage of the O2 sensor at the inflection point e4 of the output characteristics thereof are substantially the same as each other, the target value is present at the inflection point. Therefore, as described above, the control properties of a process of controlling the air-fuel ratio in the exhaust gas to keep the output voltage of the O2 sensor at the target value are improved. Consequently, the air-fuel ratio in the exhaust gas which is supplied from the internal combustion engine to the catalytic converter can stably be controlled at an air-fuel ratio for keeping the output voltage of the O2 sensor at the target value. As a result, the catalytic converter has its required exhaust gas purifying capability maintained stably. When the output voltage of the O2 sensor changes from the inflection point e4 into a leaner air-fuel ratio range due to disturbances, because the output voltage of the O2 sensor shifts into the large-gradient segment, the difference between the output voltage of the O2 sensor and the target value increases. As a result, the air-fuel ratio in the exhaust gas can quickly be brought back toward an air-fuel ratio corresponding to the target value. NOx contained in the exhaust gas, in particular, is thus quickly prevented from increasing.
The O2 sensor according to the present invention therefore has output characteristics adequate for achieving the required exhaust gas purifying capability of the catalytic converter. The air-fuel ratio in the exhaust gas can be controlled by adjusting the amount of the fuel supplied to the internal combustion engine.
The above output characteristics of the O2 sensor can be accomplished by adjusting the contents of materials that make up an active element of the O2 sensor. However, the output characteristics of the O2 sensor should preferably be accomplished by achieved by controlling the temperature of the active element of the O2 sensor at a predetermined temperature. Preferably, the predetermined temperature is at equal to or higher than 750xc2x0 C.
As the temperature of the active element of the O2 sensor is kept at the predetermined temperature, the output characteristics of the O2 sensor can stabilized into those characteristics which match the target value even if the temperature of the exhaust gas emitted from the internal combustion engine varies. As a result, the process of controlling the air-fuel ratio in the exhaust gas can reliably be stabilized, and hence the exhaust gas purifying capability of the catalytic converter can further be stabilized.
When the predetermined temperature is 750xc2x0 C. or higher, even if the temperature of the active element of the O2 sensor that is controlled varies slightly, the stability of the output characteristics of the O2 sensor is high, and the target value and the average gradient of the inflection point e4 with respect to a change in the air-fuel ratio in the exhaust gas match each other well. Stated otherwise, the sensitivity of the output voltage of the O2 sensor with respect to a change in the air-fuel ratio in the vicinity of the target value (=the inflection point e4) is not too high and too low, but is adequate. As a result, the control properties of the air-fuel ratio control process are effectively improved. When the temperature of the active element of the O2 sensor is controlled at a temperature of 750xc2x0 C. or higher, the purification optimizing output voltage Vop of the O2 sensor which optimizes all of the rates of purification of CO, HC, NOx by the catalytic converter can be present at the inflection point e4. With the purification optimizing output voltage Vop set to the target value, therefore, the exhaust gas purifying capability of the catalytic converter can stably and effectively be increased in combination with the improved control properties of the air-fuel ratio control process.
The temperature of the active element of the O2 sensor can be controlled by, for example, controlling energization of an electric heater combined with the O2 sensor in the vicinity of the active element thereof. It is necessary to recognize the temperature of the active element of the O2 sensor in order to control the temperature of the active element of the O2 sensor. The temperature of the active element may be either detected directly by a temperature sensor which is coupled to the O2 sensor near its active element or estimated based on a suitable model.
An apparatus for controlling an air-fuel ratio, a method of controlling an air-fuel ratio, and a recording medium which stores an air-fuel ratio control program according to the present invention will be described below. To achieve the above object, according to the present invention, there is provided an apparatus for controlling an air-fuel ratio in an exhaust gas supplied from an internal combustion engine to a catalytic converter disposed in an exhaust passage of the internal combustion engine for keeping an output voltage of an O2 sensor at a predetermined target value to achieve a predetermined exhaust gas purifying capability of the catalytic converter, the O2 sensor being disposed in the exhaust passage downstream of the catalytic converter for generating an output voltage having a level depending on the concentration of oxygen in the exhaust gas, the apparatus comprising sensor temperature control means for controlling the temperature of an active element of the O2 sensor so as to be kept at a predetermined temperature.
According to the present invention, there is also provided a method of controlling an air-fuel ratio in an exhaust gas supplied from an internal combustion engine to a catalytic converter disposed in an exhaust passage of the internal combustion engine for keeping an output voltage of an O2 sensor at a predetermined target value to achieve a predetermined exhaust gas purifying capability of the catalytic converter, the O2 sensor being disposed in the exhaust passage downstream of the catalytic converter for generating an output voltage having a level depending on the concentration of oxygen in the exhaust gas, the method comprising the step of controlling the temperature of an active element of the O2 sensor so as to be kept at a predetermined temperature when the air-fuel ratio in the exhaust gas is controlled.
According to the present invention, there is further provided a recording medium readable by a computer and storing an air-fuel ratio control program for enabling the computer to perform, in a system having an O2 sensor disposed in an exhaust passage of an internal combustion engine downstream of a catalytic converter disposed in the exhaust passage, for generating an output voltage having a level depending on the concentration of oxygen in an exhaust gas supplied from the internal combustion engine through the exhaust passage to the catalytic converter, a process of controlling an air-fuel ratio in the exhaust gas for keeping an output voltage of the O2 sensor at a predetermined target value to achieve a predetermined exhaust gas purifying capability of the catalytic converter, the air-fuel ratio control program comprising a program for enabling the computer to perform a process of controlling the temperature of an active element of the O2 sensor so as to be kept at a predetermined temperature when the air-fuel ratio in the exhaust gas is controlled.
According to the present invention, since the temperature of the active element of the O2 sensor is kept at the predetermined temperature, the output characteristics of the O2 sensor can be kept constant. Therefore, the output characteristics of the O2 sensor can be stabilized even if the temperature of the exhaust gas emitted from the internal combustion engine varies. As a result, the exhaust gas purifying capability of the catalytic converter can be stabilized by the process of controlling the air-fuel ratio in the exhaust gas to maintain the output voltage of the O2 sensor at the predetermined target value.
According to the present invention (the apparatus for controlling the air-fuel ratio, the method of controlling the air-fuel ratio, and the recording medium which stores the air-fuel ratio control program), the predetermined temperature is preferably equal to or higher than 750xc2x0 C. With the above predetermined temperature, the output characteristics of the O2 sensor can stabilized even if the temperature of the active element of the O2 sensor which is controlled varies slightly. As a result, the exhaust gas purifying capability of the catalytic converter can be stabilized. The O2 sensor generally comprises a sensor for generating an output voltage which changes with respect to the air-fuel ratio in the exhaust gas which is represented by the concentration of oxygen, at a gradient which switches from a large gradient to a small gradient via an inflection point e4 as the air-fuel ratio turns richer. When the temperature of the active element of the O2 sensor is controlled at a temperature of 750xc2x0 C. or higher, the purification optimizing output voltage Vop of the O2 sensor which optimizes all of the rates of purification of CO, HC, NOx by the catalytic converter can be present at the inflection point e4. With the purification optimizing output voltage Vop set to the target value, therefore, the exhaust gas purifying capability of the catalytic converter can stably and effectively be increased in combination with the improved control properties of the air-fuel ratio control process.
According to the present invention (the apparatus for controlling the air-fuel ratio, the method of controlling the air-fuel ratio, and the recording medium which stores the air-fuel ratio control program), if the O2 sensor comprises a sensor for generating an output voltage which changes with respect to the air-fuel ratio in the exhaust gas which is represented by the concentration of oxygen, at a gradient which switches from a large gradient to a small gradient via an inflection point as the air-fuel ratio turns richer, then the predetermined temperature is preferably a temperature which is determined such that the output voltage of the O2 sensor at the inflection point is substantially the same as the target value when the temperature of the active element of the O2 sensor is kept at the temperature.
By thus controlling the temperature of the active element of the O2 sensor, the target value for the output voltage of the O2 sensor and the output voltage at the inflection point e4 are substantially the same as each other, so that the target value is present at the inflection point e4. As described above with respect to the O2 sensor according to the present invention, therefore, the control properties of a process of controlling the air-fuel ratio in the exhaust gas to keep the output voltage of the O2 sensor at the target value are improved. Consequently, the air-fuel ratio in the exhaust gas which is supplied from the internal combustion engine to the catalytic converter can stably be controlled at an air-fuel ratio for keeping the output voltage of the O2 sensor at the target value. As a result, the catalytic converter has its required exhaust gas purifying capability maintained effectively and stably. When the output voltage of the O2 sensor changes from the inflection point e4 into a leaner air-fuel ratio range due to disturbances, the air-fuel ratio in the exhaust gas can quickly be brought back toward an air-fuel ratio corresponding to the target value. NOx contained in the exhaust gas, in particular, is thus quickly prevented from increasing.
In the apparatus for controlling the air-fuel ratio according to the present invention, the sensor temperature control means should preferably control the temperature of the O2 sensor at a temperature lower than the predetermined temperature until a predetermined period of time elapses after the internal combustion engine has started. Similarly, the method of controlling the air-fuel ratio according to the present invention should preferably have the step of controlling the temperature of the O2 sensor at a temperature lower than the predetermined temperature until a predetermined period of time elapses after the internal combustion engine has started. Likewise, in the recording medium storing the air-fuel ratio control program, the air-fuel ratio control program should preferably comprise a program for enabling the computer to perform a process of controlling the temperature of the O2 sensor at a temperature lower than the predetermined temperature until a predetermined period of time elapses after the internal combustion engine has started. The temperature lower than the predetermined temperature is 600xc2x0 C., for example.
With the above arrangement, even if moisture in the exhaust gas is deposited on the active element of the O2 sensor, the active element is prevented from being abruptly heated and hence from being damaged due to thermal stress or the like.
In the apparatus for controlling the air-fuel ratio according to the present invention, if the sensor temperature control means controls the temperature of the active element of the O2 sensor with an electric heater, then the sensor temperature control means should preferably de-energize the electric heater when the temperature of the electric heater is in excess of a predetermined upper limit (e.g., 930xc2x0 C.). Similarly, in the method of controlling the air-fuel ratio according to the present invention, if the temperature of the active element of the O2 sensor can be controlled by an electric heater, then the method should preferably comprise the step of de-energizing the electric heater when the temperature of the electric heater is in excess of a predetermined upper limit. Likewise, in the recording medium storing the air-fuel ratio control program according to the present invention, if the temperature of the active element of the O2 sensor can be controlled by an electric heater, then the air-fuel ratio control program should preferably comprise a program for enabling the computer to perform a process of de-energizing the electric heater when the temperature of the electric heater is in excess of a predetermined upper limit.
With the electric heater thus controlled in its energization, it is possible to prevent the electric heater from suffering a disconnection and also to prevent the active element of the O2 sensor which incorporates the electric heater from being damaged by overheating.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.